1. Field of the Invention
The present invention relates to a technique for converting input moving image data into higher frame-rate moving image data.
2. Description of the Related Art
A CRT has long served as a moving image display device typified by a television receiver. In recent years, however, a thin panel type display device using a liquid crystal device is mainly used. FIG. 10 shows a feature of the liquid crystal device. In FIG. 10, a horizontal axis indicates time, and a vertical axis, pixel brightness. The frame rate in this example is 60 frames/sec. As shown in FIG. 10, in the case of liquid crystal device, light emission is held during a period of 1/60 sec for 1 frame. Accordingly, the liquid crystal device is called a “hold type” device.
The hold type device has a problem that a blur often occurs with respect to a motion. FIG. 11 shows the problem. In FIG. 11, a horizontal axis indicates a position on a screen, and a vertical axis, time. The figure shows an example where a rectangular waveform moves rightward from a left side on the screen. When this motion is visually checked, a status where pixels stay in the same positions for 1/60 sec causes a relative delay with respect to the motion. When the hold time is long, the width of the delay is prolonged, and it is visually perceived as a moving blurring on the screen. The bottom part of FIG. 11 shows a view upon pursuit, in which a blur with a certain width is detected on an edge.
As an example of a moving blurring countermeasure, the hold time is shortened by raising a driving frequency. FIG. 12 shows an example of display at a doubled frequency, 120 Hz.
Further, to obtain a doubled frame rate, a method of dividing an input image into an image including high frequency components and an image including only low frequency components and producing a display in a time direction is known. FIG. 13 shows a dynamic characteristic of a drive-distributed image by this method. As it is understood from a comparison with FIG. 11, the moving blurring is greatly reduced.
Further, as a device having a similar light emission characteristic to that of a CRT, a field emission type display device is developed. FIG. 14 shows the light emission characteristic of such a device. As in the case of FIG. 10, a horizontal axis indicates time, and a vertical axis, pixel brightness. This type of display device, in which light emission is made only for a very short period of time within the 1/60 sec period, is called an “impulse type” device.
The impulse type device, which repeats execution/non-execution of light emission in a cycle of 1/60 sec, has a disadvantage that this turning on and off is frequently perceived as flicker. As the flicker becomes conspicuous with the display area, in the recent trend for large screen display device, the above disadvantage may cause a serious problem.
FIG. 15 shows a dynamic characteristic of the impulse type device. As the most important feature of the impulse type device, different from the characteristic of the hold type device, a moving blurring which may become an afterimage does not occur.
As an example of flicker countermeasure, the driving frequency may also be raised. FIG. 16 shows an example of display at a doubled frequency, 120 Hz. In the case of the impulse type device, a display, the brightness level of which is the half of brightness for 1 display, is produced twice, and thereby an equivalent brightness is obtained.
FIG. 17 shows a dynamic characteristic in a case where an image including high frequency components and an image including only low frequency components are divisionally displayed in a time direction. When the frames are respectively displayed twice, double vision occurs, however, by displaying only the high frequency side once, visual degradation is suppressed except for a blur due to low frequency components.
As described above, the distribution of a frame image into 2 sub frames in accordance with frequency component is advantageous as a moving blurring countermeasure in the hold type display device and a flicker countermeasure in the impulse type display device.
As an example of implementation of hold-type doubled-speed driving, Japanese Patent Laid-Open No. 2006-184896 is known. FIG. 18 shows a part of circuitry in the document. A low-pass filter processor 1002 generates a sub frame including only low frequency components from an input frame. The sub frame including only low frequency components is temporarily stored in a frame memory 1004. On the other hand, a difference detection unit 1003 subtracts the sub frame including only low frequency components generated by the low-pass filter processor 1002 from the input frame, i.e., it detects a difference, and thereby extracts high frequency components. The generated high frequency components are added to the input frame by an adder, and thereby a sub frame where high frequency components are emphasized is obtained. A selection circuit 1005 selects the low frequency sub frame stored in the frame memory 1004 or the high frequency sub frame in a cycle of 120 Hz and sends the selected sub frame to the next stage processing. By alternately displaying the sub frame without high frequency component and the sub frame where high frequency components are emphasized, an original frame image is reproduced in a view in a cycle of 60 Hz.
However, in some cases, the apparent frame image obtained by combining two sub frames is not the same as the original frame image. Hereinbelow, the problem will be described using FIGS. 19A to 19F.
FIG. 19A shows an example of a waveform of an input frame image. FIG. 19B shows an output waveform obtained by filter processing on the input frame image by the low-pass filter processor 1002 in FIG. 18. FIG. 19C shows an output waveform detected by the difference detection unit 1003 in FIG. 18. As it includes high frequency components, it has positive and negative values. FIG. 19D shows a waveform obtained by adding the high frequency components (FIG. 19C) to the original input waveform (FIG. 19A). In theory, by alternately displaying the waveform in FIG. 19C and the waveform in FIG. 19D in a cycle of 120 Hz, an apparent waveform is the same as the waveform in FIG. 19E. However, when the value of the low brightness level part in FIG. 19A is “0” or a value close to “0”, a part of the waveform in FIG. 19D has a negative value. Since it is impossible to display a negative value image, actually the negative value is displayed as “0” in the waveform in FIG. 19E different from that in FIG. 19D. In this case, as the waveform in FIG. 19B and the waveform in FIG. 19E are alternately displayed as an apparent composite waveform, a waveform as shown in FIG. 19F is perceived by the human eye. The waveform in FIG. 19F means that when white characters are displayed on black background (such as captions), the image is perceived as an image where the outlines of characters are blurred. In this manner, in accordance with waveform of input image, a distribution-processed image does not look alike an original image and is perceived as degradation.